/Engine/Source/Runtime/Engine/Classes/Engine/GameViewportClient.h
Notification of server travel error messages, generally network connection related (package verification, client server handshaking, etc) generally not expected to handle the failure here, but provide notification to the user
virtual void PeekTravelFailureMessages ( [UWorld](API\Runtime\Engine\Engine\UWorld) * InWorld, enum [ETravelFailure::Type](API\Runtime\Engine\Engine\ETravelFailure__Type) FailureType, const [FString](API\Runtime\Core\Containers\FString) & ErrorString )